Rheological Measurements by AFM of the Formation of Polymer Nanofibers Mehdi M. Yazdanapanah, Mahdi Hosseini, Santosh Pabba, Scott M. Berry, Vladimir V. Dobrokhotov, Abdelilah Safir, Robert S. Keynton and Robert W. Cohn ElectroOptics Research Institute and Nanotechnology Center University of Louisville, Louisville, KY 40292 ABSTRACT Polymer fiber can be formed by pulling a thread of polymeric liquid if the fiber solidifies before it breaks up by capillary thinning. Fiber diameter is well correlated with a processing parameter [1] that is a simple function of viscosity, surface tension and evaporation rate. Not only can the effect of the processing parameter be observed in the atomic force microscope (AFM), but the fundamental material parameters can also be determined with the same AFM setup. The usual problem with tapered AFM tips, of liquids wetting unstably up the tapered AFM tip and even onto the cantilever, is resolved by the use of long cylindrical tips of constant diameter. Previously, focused-ion-beam milled AFM tips had been used for this reason, but they are expensive and limited in length to a few microns. We recently demonstrated a straightforward method of growing Ag-Ga nanowires (100 nm diameter, 7-70 microns long) onto AFM tips at room temperature. These constant diameter nanowires are shown to give clearly measurable force-distance curves when inserted through the surface of a liquid, which provides clean measurements of surface tension, contact angle, and evaporation rate, while shear viscosity is determined through Q-damping as a function of insertion distance into the liquid. INTRODUCTION The ability to interactively draw nanometer diameter polymer fibers and perform direct point-to-point interconnections in three dimensions would be a valuable tool for the fabrication of complex nanosystems. AFM’s appear to have the necessary force sensing capabilities to enable drawing and interactive sensing of the fiber diameters and lengths—as individual fibers are being drawn. Already, nanoscale fibers have been drawn by various groups [2-6]. In this paper, we show (1) how AFM force sensing relates fiber drawing parameters to the material parameters of polymeric liquids used in making the fibers and (2) that the AFM can clearly resolve these material parameters—all in the same AFM configuration with the identical AFM probe. Significance of the processing parameter. The key material parameters of surface tension σ, shear viscosity η and evaporation rate χ correlate with fiber geometry through the processing parameter [1] σ ηχ = P . (1) Its effect on fiber length can be illustrated by retracting a nearly cylindrical, parylene-coated needle on the tip of an AFM cantilever from aqueous solutions of 1.0 x 10 6 gm/mol molecular weight poly(ethylene oxide) (PEO). The concentration range corresponds to changes in processing parameter of around six orders of magnitude. Figure 1a shows the transient forces measured by the AFM for a linear retraction rate of ν=2 μm/s. At zero force the liquid thread breaks apart, which is reported as breakup time t b in Fig. 1b. The thread length at breakup corresponds to k t F vt l b b ) ( 0 + = (2) where the second term F(t o )/k accounts for the initial deflection of the cantilever when retraction starts. F(t o ) is the initial force on the AFM cantilever and k is the cantilever spring constant. Fig. 1b shows that the length and time at fiber breakup increases as l P l l o b ∆ = (3) t P t t o b ∆ = (4)